This invention relates generally to cable-ready television receivers and specifically to an intermediate frequency (IF) circuit that is alterable for operation with cable signals and off-air signals.
In intercarrier sound television (TV) receiver systems, the IF circuits process a video carrier of 45.75 MHz, an audio carrier of 41.25 MHz and a 3.58 MHz color subcarrier, which is modulated on the video or picture carrier. As is well-known, when detecting the IF video carrier signal the audio carrier must be suppressed to a low amplitude with respect to the video carrier to avoid cross modulation between carriers, especially between the audio carrier and the color subcarrier. Since the audio carrier must also be recovered, however, it cannot be suppressed to the point that its detection is impaired. Conventional IF circuits include an adjustable audio carrier trap which, by appropriate tuning of its frequency and impedance at resonance, tailors the response characteristic of the IF amplifier circuit at the audio carrier frequency to that which is desired. These techniques are well-known in the art and their use generally provides excellent performance.
In cable television systems, a single coaxial cable carries a multiplicity of TV channels of differing frequencies. The amplifiers at the cable head-end or transmitting station are of necessity wide-band, which enhances the probability of interference between the various carriers present in the TV signal. In an effort to reduce the probability of intermodulation distortion and also to conserve transmitting power most cable systems reduce the audio carrier amplitude with respect to the video carrier amplitude to a greater degree than is their practice with respect to over-the-air signals, hereinafter referred to as "off-air" signals. For example, in a conventional off-air TV signal, the audio carrier is from 7 to 10 db below the picture carrier, whereas in a cable system the audio carrier may be from 17 to 20 db below the picture carrier. These conflicting signal arrangements pose a dilemma for television receiver manufacturers since conventional TVs include a trap at the audio carrier frequency to keep the level of the audio carrier down with respect to the video carrier in the IF signal.
The problem is that most receiver manufacturers produce "cable-ready" TV receivers in which standard VHF and UHF channels as well as cable channels that extend from the VHF television frequency range up into portions of the UHF range, are tunable. Some type of switching is generally incorporated on the front panel of such TVs to enable the user to select between cable and off-air signal reception. Consequently, the TV may be used with either (or both) off-air and cable signals. As will be seen, the audio trap poses a problem with cable signals.
Another advance in TV performance has been brought about by the use of surface acoustic wave (SAW) filters in IF circuits. The advantages of a SAW filter IF stage are extreme reliability, consistent performance among units and exceptional stability. A major disadvantage of a SAW filter is that it is quite lossy. Therefore, it is desirable that the audio carrier in a cable television IF signal not be further diminished before application to a SAW IF input.